1. Field
The disclosure relates to a brake system for a vehicle. More particularly, the disclosure relates to an electro-mechanical brake system capable of braking a vehicle through the operation of an electric motor and a reduction unit.
2. Description of the Related Art
A brake system is installed in a vehicle to reduce the speed of the vehicle, to stop the vehicle or to maintain the vehicle in a parking state.
In general, a friction type brake system is mainly used as the brake system. The friction type brake system converts dynamic energy into thermal energy by using frictional force and discharges the thermal energy into the atmosphere to perform the braking operation. According to the friction type brake system, pads are hydraulically pressed against both sides of discs rotating together with vehicle wheels, thereby braking the vehicle.
However, according to the hydraulic brake system of the related art, the pads are intensely pushed toward the discs by the hydraulic pressure upon the braking operation, so the hydraulic brake system requires the complicate structure including a master cylinder operated by a booster, which boosts pedal force, to generate the hydraulic force, hydraulic lines extending to a wheel cylinder, and various devices for controlling and supporting the above components. Due to the complicate structure and use of the hydraulic pressure, the hydraulic brake system represents limitations in terms of the reliability of the braking performance and stability.
For this reason, an electro-mechanical brake (EMB) system capable of simplifying the structure and improving the reliability of the braking performance has been extensively used.
According to the EMB system, an electronic control unit (ECU) receiving electronic pedal information upon the braking operation drives a motor to generate axial driving force, so that a piston is moved in the axial direction through a reduction unit coupled to the motor. Thus, pads are pressed against discs, thereby performing the braking operation.
The motor, which is a main actuator of the EMB system, may include a BLAC type motor or a BLDC type motor. In addition, a cascade controller including a position controller (or power controller), a speed controller and a current controller can be used in order to control the motor.
If an operation command is generated when the motor of the EMB system is not operated, an operation delay may occur due to the initial inertia and stick slip. In this state, if the motor is operated, the speed controller outputs a great command signal value due to the great error caused by the operation delay, so that the overshoot of the motor may occur. If the overshoot of the motor occurs, the motor may rotate faster than the command speed of the motor controller, so the speed controller outputs the reverse command to the motor (that is, in the direction to stop the operation of the motor) in order to reduce the speed of the motor. In general, the above procedure is regarded as the correct control behavior in view of the control procedure. However, in view of the EMB system, since the motor just starts to operate, the pad does not make contact with the disc, so that the braking force is not yet generated. Thus, the motor ought to be driven approximately to the maximum speed even if the speed error (greater than the command speed) occurs. In detail, speed reduction of the motor ought to be absolutely forbidden.